The present invention relates to dynamoelectric machines, particularly dynamoelectric machines free of ripple and other discontinuities over a limited arc range up to and greater than 180 mechanical degrees.
In the industry of high performance position control there is a need for rotating electromagnetic components which produce essentially ideal performance over a limited angular excursion. FIG. 1 shows a prior art dynamoelectric device 10 commonly known as a toroidally wound, limited angle device. It includes a permanently magnetized rotor 12 having a north pole N and a south pole S. Rotor 12 is surrounded by a highly magnetically permeable stator core 14 (typically soft iron) on which is wound a coil 16 of matching polarity as that of the rotor 12. The principle of operation of these devices is well known in the art. An air gap is present between the stator core 14, which forms the flux return path, and the polarized ends N, S of the rotor 12. A high flux density (B) is established in this gap that "cuts" the turns 16 of the winding. Fundamental equations describing the produced torque and voltage are as follows: EQU Torque=N(I.times.B)l EQU Voltage=NBlv
where:
I is the current (in Amps) flowing through the coil 16.
l is the mutual length of rotor 12 and stator 14 (into the page).
v is the velocity of the rotor 12.
B is the flux density in the vicinity of the windings (air gap)
N is the number of series wound turns 16 "cut" by the flux (B) in the vicinity of the turns 16 in the air gap between the turns 16 and the polarized ends N, S of the rotor 12.
X designates the vector cross product.
FIG. 2 graphically shows the typical performance of dynamoelectric device 10. Axis 18 represents torque produced when the dynamoelectric device 10 is operated as a motor and axis 20 represents voltage, or Emf (electromotive force), produced when the dynamoelectric device 10 is operated as a tachogenerator. Axis 22 represents angular displacement of rotor 12 in degrees, with zero degrees being where the north pole of rotor 12 of FIG. 1 is at its furthest left-hand point (nine o'clock position). A distinct characteristic of dynamoelectric device 10 is its relatively trapezoidal Emf/Torque profile. Since device 10 has 2 poles, the waveform 24 inverts polarity at two places, 180 degrees apart. Because magnetic poles are always found in pairs, north and south, it has been difficult to extend the range of constant torque beyond 180 degrees.